1. Technical Field
The present disclosure relates to a semiconductor light source lighting circuit which applies a drive voltage to a semiconductor light source, such as a Light Emitting Diode (LED) and the like, and a vehicular lamp which includes the semiconductor light source lighting circuit.
2. Related Art
In recent years, an LED which has a longer lifetime and lower power consumption, instead of a halogen lamp which has a filament of the related art, is used in a vehicular lamp such as a headlight and the like. The intensity of LED emission, that is, brightness, is dependent on the magnitude of a current flowing in the LED, such that a lighting circuit for adjusting the current flowing in the LED is needed when using the LED as a light source. Such a lighting circuit usually has an error amplifier to perform feedback control so that the current flowing in the LED is constant.
Electricity to a lighting circuit is supplied from a normal vehicle battery. Since the vehicle battery supplies electricity to various portions of a vehicle in addition to the lighting circuit, such that a change in a battery voltage is relatively large. When the battery voltage changes across the forward dropping voltage of the LED, it is preferable that the lighting circuit have both a function of stepping up a battery voltage and a function of stepping down the battery voltage.
JP-A-2005-198410 discloses a DC/DC converter of a step-up and step-down chopper type. In the converter, a p-type metal oxide semiconductor field effect transistor (hereinafter, referred to as p-type MOSFET) is adopted as a high-side switching element.
In an LED which is used as a light source of a vehicular lamp, generally a relatively large current flows so as to realize a required brightness. Therefore, when using the DC/DC converter of the step-up and step-down chopper type to light the LED, it is preferable to use, in general, an n-type metal oxide semiconductor field effect transistor (hereinafter, referred to as n-type MOSFET) which has a lower on-resistance than the p-type MOSFET as a high-side switching element.
In order to turn on the n-type MOSFET, a gate voltage needs to be higher than a source voltage. In response to this, a bootstrap circuit which generates a necessary gate voltage by using ON and OFF operation of the n-type MOSFET itself is known. In addition, in a voltage step-up mode of the voltage step-up and step-down converter, a high-side n-type MOSFET has to be on all the time, such that a charge pump for charging a bootstrap capacitor in the voltage step-up mode is considered to be additionally provided.
However, in the above configuration, when the input voltage to the voltage step-up and step-down converter is lowered, even if the voltage step-up and step-down converter needs to operate in the voltage step-up mode, the charge capacity of the charge pump is lowered along with the lowering of the input voltage.